The vertebrate retina changes many aspects of its physiology with respect to time of day, which allows the retina to adapt efficiently to varying light intensities of many orders of magnitude. These changes include retinomotor movements, visual sensitivity, synaptic function, synthesis and release of neuromodulators and gene expression. Although it is known that vertebrate retinas contain endogenous circadian clocks that control these physiologies, little is known about how these clocks transmit this temporal information to the retinal cells. A powerful model system for studies of retinal and clock function is Xenopus laevis. The eyes from these frogs are very robust in culture for many days and maintain most aspects of normal physiology, including rhythmic physiologies of many types. Furthermore, new technology now allows the production of transgenic frogs providing the opportunity to perform precise mechanistic studies of these cellular functions in vivo. Previously, my lab isolated a novel gene named nocturnin that is expressed for only a few hours in early night in the photoreceptor cells of the retina. This gene encodes a protein that resembles a transcriptional coactivator and our hypothesis is that this gene product is involved in regulating night-time events in the retina. The first aim of this proposal continues the characterization of the nocturnin protein, investigating its intracellular location, its phosphorylation status and its turnover rates. The second aim will focus on identifying proteins that interact with nocturnin, through biochemical means and yeast two-hybrid screens. The third aim will address the role of nocturnin within the photoreceptors by producing transgenic frogs with various gain or loss-of-function mutations of the nocturnin gene. Finally, the fourth aim will continue our studies on the nocturnin promoter, by identifying the elements and proteins involved in its precise temporal regulation. Our overall goal is to provide data about nocturnin that links the central circadian timekeeping mechanism to the cellular rhythms that are so important for the normal homeostasis of the retina.